1. Field of the Invention
The present invention relates generally to a method and apparatus for Rapid Freezing Prototyping and more particularly to a method and apparatus for building ice patterns according to a predetermined design by selectively depositing and freezing water layer by layer in a controlled manner.
2. Description of the Relevant Art
Presently, there exists-three general categories which mankind has used for tool building. One of the earliest and most popular categories is known as machining, which includes, for example, milling, lathe cutting, drilling, grinding, flame cutting, and electric discharge machining. The common feature found in this category is material removal. All machining methods begin with work pieces in which the shape of the part is achieved by either manually or automatically controlling the tool path relative to the work piece to remove material from the piece until the desired result has been reached. The second category for making parts is forced forming, for example by injection molding, various kinds of casting and forging. The common feature of this method is that a cavity or mold corresponding to the shape of the part to be made in which the shape of the part is accurately or near accurately copied from the shape of the cavity or mold by making the material flexible. The third category for making parts utilizes solid free form fabrication (SFF), also known as rapid prototyping, which emerged in the mid 1980's. The common feature of SFF is that material added in a layer-by-layer manner until the shape of the part to be built is achieved by accurately controlling the building paths of layers.
Both machining and shape copying methods are methods that are particularly well suited for large-scale production. However, where only a small number of parts are desired (e.g., 1-10), such conventional methods are deficient because they usually involve a large initial expense for labor, setting up of the machining protocols, tools or molds. Furthermore, because of complexity limitations in machining, a very complex part must usually be divided into several segments that are built separately and then later assembled.
To overcome these difficulties SFF processes have been developed in which the cost of part building does not depend on the quantity and even the complexity of the part. It is especially suitable for new product design and development, such as design visualization, form fit, functional testing and rapid tooling in order to make fully functional parts. Conventional SFF processes include Stereolithography, Laminated Object Manufacturing, Fused Deposition Modeling, and Selective Laser Sintering. These SFF processes all use the same concept of building three-dimensional objects in a layer-by-layer manner. However, the methods for building each layer and the materials required are different for each type of SFF process.
Some known SFF processes are disclosed as follows:
In the U.S. Pat. No. 4,575,330, an ultraviolet (UV) laser beam is focused on the surface of liquid photo curable resin. The resin exposed to UV laser radiation will change rapidly from liquid to solid state. The beam is accurately directed by the computer according to the cross-sectional information of the CAD model. However, the curable photopolymer used in the process is hazardous. Casting with the patterns made by this technology is somewhat difficult because the cured material expands and evaporates hazardous fumes rather than melts when heated.
In U.S. Pat. Nos. 5,121,329 and 5,340,433, a non-laser SFF process is disclosed that uses a heated working head to melt and deposit the material onto a substrate to build a part. The material comes out of the nozzle of the head and then rapidly solidifies and adheres to the previous layer or substrate. The temperature of the heated material and the ambient is precisely controlled in order to make the deposited material solidify very rapidly. Wax and ABS plastic are found to be especially suitable for this process. However, plastic is not suitable for tooling. Wax patterns can be directly used for investment casting, but the accuracy of the wax patterns is not as good as plastic patterns. Moreover, this process has some difficulties in controlling the heat accumulation and layer bonding. The build speed is also very slow as compared to other SFF processes. Higher building speed usually results in more serious heat accumulation, worse accuracy and surface finish.
In U.S. Pat. No. 4,863,538, a sintering process (called SLS), uses a CO.sub.2 laser beam directed to selectively sinter a powder according to cross-sectional information provided by a computer. This process has the potential to build parts with a wide variety of materials, e.g., plastic, wax, ceramic and metal, etc. However, this process also has heat accumulation problem, especially when building parts with a high melting point material. Currently, in order to build metal parts with this technology, high temperature post sintering or metal infiltration is necessary. However, these kinds of post sintering techniques are believed to cause part shrinkage or loss of surface finish.
In U.S. Pat. No. 4,752,352, another laser layered building process is introduced using a CO.sub.2 laser beam that is directed to cut contours on a sheet of material to make each layer of a 3D solid model. The layers are bonded together with adhesives coated on the back of the sheet material. The main disadvantage of this process is that it is not convenient to make metal functional parts with the patterns made of this technology. Furthermore, the cutting process produces a lot of hazardous smoke and results in a surface finish that is also poorer as compared to SLA parts.
Broadly speaking, the current SFF processes have made great successes in rapid prototyping as well as rapid tooling. Layered material additive manufacturing has been rapidly taken from a concept to an established commercialized manufacturing alternative. However, to further improve the industrial application of this technology, some basic problems need to be solved, such as the high cost of SFF machinery and material, poor part accuracy and surface finish, process or material pollution, and difficulties in making metal parts or molds with the SFF prototypes.
Thus, a need exists for reducing system cost, improving building speed, accuracy, surface finish and also for developing new SFF processes that are cleaner to the environment.